[SI-LIST] Re: backplane PDN/EMC challenges

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: qantrix <qantrix@xxxxxxxxx>
  • Date: Tue, 22 Feb 2011 22:29:36 -0800

Inserting magnetics between Vss used as high speed signal return and 
power common that returns to chassis creates lots of opportunity for 
resonances in the 100MHz and higher range due to the shunt capacitance 
between the Vss planes, the power common and the chassis.  A CM choke 
can aggravate DC current crowding and sneak path through signal common 
connections.  DC isolation between signal Vss and power supply common 
requires isolated voltage conversion.

The bottom line is you need to determine what is required to meet 
applicable safety standards for your equipment.  If those can be 
satisfied with common signal and power commons, then the simpler 
arrangement of DC connection between power and signal return almost 
always leads to fewer and more manageable problems.

Steve.


qantrix wrote:
> Hi All ,
> we are challenging how to provide power/return to cards connected to
> backplane
> and how to deal with chassis/signal ground
>
> So device will transfer high speed signals with Tr=0.1ns. Digital video data
> rates
> General curretn consumption will be 12V 40A. So like 5amps for each of
> cards(8).
> Device doesnt have high voltages or high energy voltage sources. general
> digital video application.
>
> There is two options - do separate  polygons for each of VCC_1...8 and  its
> returns by using 10A common mode choke placed  at power source.
> Doing that supply and return currents will be under control and no return
> current hot spots will be produced. and do it for all plug in cards.
>
> Second option is doing common 12V polygone and return share with
> signal(highspeed ) gnd - there will be a lot of gnd planes in 14 layer
> stackup.
> On plug in cards also - can be two strategies - separate polygones(control
> for supply/return currents)  or common - current will return with signal
>  return in connector)
>
> First option is rigth from general understanding point. But in practice - do
> the second variant work well? What says experience?
>
> Another question - what is common practice for chassis gnd in rackmount
> devices - does in need to be dc separated by caps from sgnd?
> Many people suggest to not separate at all  - that will give good emc
> perfomance - all noisy gnds will be bonded with minimum inductance to metal
> box(faraday cage) also esd current will have low impedance return path.
> Other people say that noisy gnd planes will drive connectors area and cables
> will act as efficient antenna.
>
> I'm betweeen two opinions, by my self i think common plane for two problems
> is good and simple way to go.
>
>
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-- 
Steve Weir
IPBLOX, LLC 
150 N. Center St. #211
Reno, NV  89501 
www.ipblox.com

(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax


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